1. Technical Field
The present invention relates generally to photolithography and more particularly to patterning different types of features.
2. Background Art
Integrated circuits are now used in almost every type of electronic product ranging from toys to massive computers. These integrated circuits are all generally made by a photolithographic process, which involves manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”.
A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material or photoresist on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer, which is subsequently cut up to form the integrated circuits. In addition to repeatedly “patterning” features onto the photoresist on the silicon wafer with a single mask, multiple masks are used to pattern different photoresist layers to form different structures at different levels on the silicon wafer.
In conventional industry practice, the masks are fabricated starting from an initial mask blank, which is transparent to the imaging light. Typically, the mask blank consists of fused silica or quartz. The mask blank is coated by an opaque film, typically a chromium based material. The opaque film is also processed using another mask and a photoresist to create openings in the opaque film to expose and permit light to pass through the openings and through the transparent quartz.
As the size and density of features start to be below the wavelength of the light used to pattern the images, a different class of masks is necessary because the light is subject to diffraction and interference effects. Diffraction effects are due to the wave nature of light, which cause peaks and valleys to occur in the intensity of light passing through an opening, such as an opening in the opaque film, and falling on the photoresist of the silicon wafer. Interference effects occur with side-by-side openings where the peaks and valleys of the light wave can interfere so as to cancel each other out or can reinforce and amplify each other depending on the location of the openings.
This is a problem when the openings are used to pattern features having sizes below the wavelength of the light used and near the resolution limit of the light. Optical distortion becomes extremely high and the correspondence between an image on the mask and the feature on the photoresist is no longer one-to-one because of information loss. This is particularly problematic where different types of features are in the same integrated circuit, such as closely spaced repeating features and spaced-apart non-repeating features.
Solutions to this problem has been long sought but has equally long eluded those skilled in the art.